It is well known that air flow through a gas turbine engine generates axial loads on rotors in gas turbine engines and have a strong influence on the life of a rotor's axial thrust bearings. Thrust loads are determined by internal engine air flow, internal engine compartment air pressures, seals and seal locations, as well as airfoil aerodynamic loads. All are integral components of the summed load on the rotor bearings and are configured to load the axial bearings only within a selected range. Advanced engine designs endeavor to reduce the magnitude and range of the bearing load in order to achieve lower bearing weights and longer bearing lifetimes.
The conventional method for measuring axial thrust loads on rotor bearings requires special load cell support rings to be incorporated into the thrust bearing support structure. As is known in the art, each separate engine type must have that bearing support structure modified to receive a specifically designed load cell support ring. The support ring is installed only for test purposes and must be removed before placing the engine in service. This measurement technique is elaborate and very expensive and is not available on engines in service.
The special instrumentation previously required to measure axial thrust load on rotor bearings includes mechanical strain gauges that are configured into a special bearing support ring. The support ring must be physically installed in the bearing assembly during thrust load measurements and removed prior to placing the engine in service. The physical installation of the instrumented bearing support ring required additional radial clearance of the bearing outer race. This alters the rotor system dynamics response and may limit the operating envelope of the engine. Smaller engines present a more difficult problem since the clearances within these engines are themselves smaller which makes load deviations more sensitive to manufacturing tolerances. Also, small engines have less available space for instrumentation. Moreover, the thrust loads measured by these strain gauges are small enough in magnitude to be outside the load range of conventional mechanical load cell techniques. In addition, known methods for measuring axial thrust loads must further differentiate between bearing loads induced by axial thrust and thermally induced loads.
It would be advantageous to have a method and apparatus incorporated into the design of a gas turbine engine so that the axial thrust loads on the rotors of an engine are established as part of the engine calibration. In addition, it would also be desirable to have a method and apparatus which can dynamically adjust rotor axial thrust loads as part of the engine scheduled maintenance to compensate for engine deterioration and seal wear. The present invention is drawn to such a method and apparatus.